Wind power generation has received a major impetus due to ever increasing demand for energy, depleting fossil fuel reserves and environmental benefits in particular with respect to the emission of greenhouse gases.
Wind turbines convert wind energy into electrical power. A wind farm or wind park, also known as wind power plant, is a collection of a few tens or a few hundreds of wind turbines installed in close vicinity with respect to each other. Within a wind farm the electric power generated by the various wind turbines is aggregated at a common collector bus (bus bar), which hereinafter is also denominated a Point of Common Coupling (PCC).
The electric power aggregated at the common collector bus has to be fed into a power grid for transmission and distribution to electric load centers and utilities located typically far away (e.g. hundreds of km). This requires that the wind farm is connected to the power grid by means of an electric high power connection. A cable transmission is required (a) for submarine power transmission (in case of offshore wind farms) and (b) for underground transmission onshore so as to connect the common collector bus to a sufficiently strong point in the grid. High Voltage Alternating Current (HVAC) and High Voltage Direct Current (HVDC) are the two alternatives for the connection of the wind farm to the grid as shown in the publication “Wind Farm Grid Integration Using VSC Based HVDC Transmission—An Overview”, S. K. Chaudhary, R. Teodorescu, P. Rodriguez, IEEE Energy2030, Atlanta, Ga. USA, 17-18 Nov., 2008.
At present most of the wind farms are connected to a main land substation of a power grid via an AC cable carrying the electric power. However, not only the active power but also the reactive power caused in particular by the capacitance of the AC cable creates an electric loss as the total current is comprised by a real and imaginary part.
WO 2010/086071 A1 discloses a method for controlling a HVDC link with Voltage Source Converters (VSC) and interconnecting two electric power systems (e.g. a wind turbine and a substation of a power grid). A model-predictive control with a receding horizon policy is employed for controlling the outer loop of a two-loop or two-layer control scheme or setup for the HVDC link. The two-loop control scheme takes advantage of the difference in speed of the dynamics of the various system variables of the HVDC link and the interconnected power systems.
Asea Brown Boveri (ABB) has published an article with the title “HVDC Light in wind farm applications”. In this article, which was available e.g. on Dec. 23, 2010, it is proposed to connect a wind farm with a power grid by means of both a HVAC link and a HVDC link. Energy losses being related to the transmission can be reduced by using the HVAC link at low wind speeds (e.g. 60% of the time) and both links, HVAC and HVDC, are used at high wind speed. In this way an optimum of total losses and system performance is obtained at all wind conditions.